This invention relates to the recording of video signals on cinematographic film.
Video signals are electrical signals representing pictures, in color or monochrome, with more than two brightness levels. The video signals would be typically derived from a television camera, a video tape recorder, or any other recording medium other than photographic film. One reason for transferring the images from other media to film is to enable the distribution of programs to the film theater. At the present time there are three methods available for the transfer of video signals to film.
These are in brief, cathode ray tube, laser scan, and electron beam scan. When using cathode ray tubes (CRTs), one has the choice of using a single tube with color filters and exposing each color separately, but this takes three times as long, or using three tubes to expose all three colors simultaneously, but then the color registration of the three images becomes a problem. Other problems include the variation of transfer characteristics between the drive to the electron guns and the light coming from the phosphor, the inherent undesirable afterglow characteristics of the phosphors, the variation of spot size with beam current, and deflection defocusing. The many variables are not conducive to reliable results.
Some improvement has been achieved by using one monochrome display and recording the red, green and blue components separately on monochrome film stock. The three prints obtained are then printed on to one film base, either using color film or by the dye transfer process. This type of system removes the major cause of problems in registering the three color images on the display, gives a wider choice of phosphor since only one is used which can be of any color to which a monochrome film will respond and avoids the problems arising from differing phosphor characteristics. On the other hand the program material has to be recorded so that it is available at three separate times for recording the red, green and blue signals, the transfer time has been tripled and problems of long term drift have become more severe.
The laser scanner is very complex because of the difficulty in deflecting laser beams at very high speed whilst maintaining sufficient resolution. The only practical method at the present time is to use a multifaceted polygon rotating at a very high speed with gas bearings. This is an extremely high precision mechanical assembly. With such systems the period during which each elemental area of the film is exposed to light is very short by comparison with normal film exposure; this tends to result in anomalous film characteristics due to reciprocity failure.
The electron beam scanner is even more complex than the laser scanner, and also uses separate passes to record three monochrome films representing the red, green and blue components of the signal. This method overcomes all the problems of phosphors since the film responds to the direct impact of the electron beam. However, the recording process including the film being scanned by the beam needs to be in vacuum to avoid problems arising from ions produced by the electron beam acting on residual gases, released particularly by the film stock. This is probably the most reliable method of transferring television programs to film but the disadvantages of sequential recording are exacerbated by the time taken to achieve an adequate vacuum, whenever the film stock reels have to he changed.
All the above have problems arising from the change of cut-off point of electron guns and also need stable high voltage supplies. A small drift of the cut-off point applying to one component can have a noticeable effect on the color balance in the shadow areas causing a color cast. Similar changes to the three components can cause noticeable changes of contrast ratio.